Maclagan, the fifth son of a distinguished Scottish physician David Maclagan (1785–1865)[2] was born in Edinburgh in 1826, and educated at the Royal High School.[3] His elder brother was the surgeon and scholar Douglas Maclagan. He served five years in the Indian Army rising to the rank of lieutenant and resigning on grounds of ill health.

In 1852, he enrolled at Peterhouse, Cambridge, where he received a degree in mathematics four years later.[4] He was immediately ordained a Deacon, and served in the Church of England thereafter. In 1869, he was Rector at Newington, and in 1875, he was Vicar of St Mary's Abbots, Kensington; both parishes being in London. During this period, he composed several hymns. On 24 June 1878, he became Bishop of Lichfield, in the same year that he made a prestigious second marriage.

In 1891 (possibly 28 July 1891), he was translated Archbishop of York, which position he held for the next seventeen years. He was appointed to the Privy Council after the accession of King Edward VII 24 January 1901.[5] He made a private visit to Russia in 1897[6] and[7] in the same year, he tried to create two new bishoprics, one in Sheffield. To do this, the Archbishop was prepared to surrender two thousand pounds of his considerable income – one thousand pounds for each new diocese, but the project still came to naught. Maclagan complained that from 1891, he had been more Bishop than Archbishop owing to the large population and territory of the diocese. In 1906, he revived the idea, specifically naming Sheffield and Hull as the preferred seats for the new dioceses. By the end of his tenure, there were still only nine dioceses in the province.[8] Sheffield did not get its own Bishop until 1914.

Maclagan was apparently a strong High Churchman, but his private beliefs had to be subsumed often. In 1899, he sat assessor with his ecclesiastical superior Dr. Frederick Temple, Archbishop of Canterbury (d. 1902), when the decision was given against the use of incense and other ritualistic practices, and was obliged loyally to uphold the primate's opinion.

Maclagan resigned his office in 1908, possibly on grounds of ill health. Archbishop Maclagan died in London on 19 September 1910, and was survived by his second wife Augusta (1826–1915).

Maclagan was twice married. His first wife was Sarah Kate Clapham (1836–1864),[9] whom he married in 1860 at the age of 34. By her he had issue two sons, Cyril and Walter.

He was married secondly on 12 November 1878, as Bishop of Lichfield, to the Honourable Augusta Anne Barrington (1836–1915), a daughter of the daughter of the 6th Viscount Barrington.[10] (Augusta Maclagan had money settled upon her when she married Maclagan, then Bishop of Lichfield, in 1878; for the sources of this money and how it was invested, see this paper.[11] About half her money was settled upon her son Eric when he married in 1913. Thus, the Archbishop's wife, son and daughter-in-law all had independent means, necessary to preserve their social status.). By his second wife, he had issue Eric (1879–1951), and a daughter Theodora "Dora" Maclagan (1881–1976).

His eldest son Cyril died childless. The second son Walter Dalrymple Maclagan (1862–1929)[12] had a son William Dalyrmple Maclagan, schoolmaster, and a daughter Dr Evelyn Maclagan,[13] both of whom apparently died unmarried. His third and youngest son Sir Eric Robert Dalrymple MACLAGAN CBE, FSA (1879–1951) married in 1913 and left issue, two sons Michael Maclagan (14 April 1914 London – 13 August 2003 Oxford), herald and historian; and Gerald Maclagan (d. 1942, killed in action) who had been working in Rhodesian Railways. His posterity is represented by the three surviving children of Michael Maclagan (d. 2003).

Maclagan was the younger brother of Professor Sir Douglas Maclagan, MD, otherwise known as Andrew Douglas Maclagan (1812–1900)[14] and[15] Sir Douglas, also educated at the Royal High School in Edinburgh, was a fellow of the Royal College of Surgeons of Edinburgh, 1833 and was knighted in 1886. He was a correspondent of Charles Darwin. Another brother was General Robert Maclagan, R.E. (1820–1893).[16] The artist Philip Douglas Maclagan (1901–1972) is descended from an older brother.

How, Frederick Douglas. Archbishop Maclagan: Being a Memoir of the Most Reverend the Right Honourable William Dalrymple Maclagan, D.D.,Archbishop of York and Primate of England London: W. Gardner and Darton, 1911.